Generally, window fog detecting apparatuses for vehicles are mainly classified into a humidity detection type and an optical type. The window fog detecting apparatus of the humidity detection type judges whether or not fog occurs at a window glass of the vehicle by comparing a glass temperature with a dew-point temperature of ambient air thereof. In this case, the dew-point temperature is calculated based on outputs of a humidity sensor and an air temperature sensor, which are arranged in a passenger compartment of the vehicle.
The glass temperature can be detected through a temperature sensor (i.e., contact detection means) mounted at an inner surface of the window glass, or an infrared sensor (i.e., non-contact detection means), or the like. Alternatively, the glass temperature can be also calculated (i.e., estimation means) according to, for example, a vehicle exterior temperature, a vehicle speed and a vehicle interior temperature.
In the window fog detecting apparatus of the humidity detection type, a relative humidity of interior air is converted into a relative humidity (glass surface relative humidity) at the glass temperature so as to judge a fog occurrence at the window glass, without comparing the glass temperature with the dew-point temperature.
The window fog detecting apparatus of the optical type detects the fog occurrence at the inner surface of the window glass by using an optical sensor in a non-contact means. For example, referring to JP-59-108939-A, a light receiving unit and a light emitting unit are provided to detect a reduction of direct reflection light due to a fog occurrence at the window glass. Referring to JP-2000-296762-A and JP-5-294139-A, the fog occurrence at the window glass is detected by sensing an increase of scattered reflection light due to fog at the window glass. Furthermore, referring to U.S. Pat. No. 6,097,024 (JP-2004-212404-A), the window fog detecting apparatus of the optical type judges the fog occurrence at the window glass by an image processing operation.
In these cases, a dehumidifying operation (operation of compressor) of a refrigerant cycle for an air conditioner of the vehicle is restricted in such a range that fog does not occur at the window glass. Therefore, the operating ratio of the dehumidifying operation is reduced so as to lower the compressor power. Thus, the fuel consumption of a vehicle engine which drives the compressor is decreased. Moreover, the vehicle is provided with the window fog judging operation and a control for heightening the window fog-preventing performance of the air conditioner, in order to prevent fog from occurring at the window glass.
Furthermore, at a low temperature in winter or the like, the inside air ratio of suction air in the air conditioner is increased within such a range that fog does not occur at the window glass. Thus, the ventilation heat loss is reduced and the heating performance of the air conditioner is improved.
However, in the window fog detecting apparatus of the humidity detection type, because there exist the detection accuracy variation and the durability deterioration of the humidity sensor, a large safety rate must be set with respect to the output value of the humidity sensor for the window fog judging operation in order to control to prevent fog from occurring at the window glass. Therefore, the above-described reduction effects of the compressor power and the ventilation heat loss cannot be sufficiently achieved.
Moreover, the window fog detecting apparatus of the optical type can only detect fog occurring at the part of the window glass where the sensor is mounted, although the window fog detecting apparatus of the optical type has a detection accuracy higher than that of the humidity detection type. Thus, fog may occur at other part of the window glass where the sensor is not arranged. Because the position where fog starts differs in response to vehicles, it is cumbersome to determine the sensor installation portions when the vehicles are developed.